Thanks for looking so thoroughly at this. This is going wrong in a couple of places.
Yet in your sheet the principal decreases. Could somebody enlighten me please?
I assume in the spreadsheet that the interest is automatically deducted from your checkings account, and end-of-year, you move saved money towards the principal. Thus the principal is decreased. This ignores a lot of stuff like the interest you gain in your savings account and what have you.
Secondly, my mortgage consists of two parts, an interest-only part, which is obviously quite simple. The second part is a complicated (perhaps typically Dutch) construction where you save into a linked savings-account and only at the end of the 30-year (or whatever) period, fully pay off the loan. Thus you maximize the fiscal benefits.
I attempted to clean up my spreadsheet but in doing so, forgot that mine actually models the two parts.
I'm doubting whether to add this construction into the spreadsheet, because it's really very specific. I could adapt the spreadsheet to show a typical annuity-based mortgage, but for me personally, it's worthless so I won't. I could also adapt it to an interest-only spreadsheet, which I've done.